1. Field of the Invention
The present invention relates to a propulsion system of displacement and semi-displacement crafts, where by displacement craft it is meant a craft that, along axes perpendicular to the water plane, is subjected only to buoyancy and not to the lift generated by the in-water motion of the craft. By semi-displacement crafts it is meant crafts in which said lift has an effect lower than said buoyancy and in which the bottom, at the speeds of design, is intended to remain immersed. Such crafts, utilized for shipping cargoes of considerable or anyhow high tonnage, are characterised by medium-low or low navigation speeds.
Such crafts differ from those whose trim is variable, i.e., markedly influenced by the lift generated by in-water motion, and in particular from water-planing ones, with the bottom completely emerged or only partially submerged. These crafts generally display at least medium-high navigation speeds.
2. Description of the Prior Art
Modern naval propulsion envisages the use of propellers, paddle wheels or jet systems.
For crafts having, in navigation, a water-immersed hull, the most widespread system utilizes fully immersed propellers, usually arranged at the craft stern. They envisage an at least partially immersed shaft line, coming out of the hull, entailing a series of complications of constructive and fluid-dynamics nature to support the driving gears, i.e., hull appendages such as shafts and shaft supports. This propulsion has become established in decades of naval technology with propulsion efficiencies generally deemed acceptable, in particular for low-speed crafts.
However, said efficiency does not appear to be substantially improvable anymore, save with the adoption of different propulsion systems.
PCT Appln. Pub. No. WO-01/47770 (Fle{hacek over (s)}) describes the use of paddle wheels for naval propulsion, with axis of rotation parallel to the longitudinal direction of the craft, i.e. the direction of motion.
Incidentally, by paddle wheel it is meant a rotary device in which the overall diameter is covered more by the diameter of the hub and of the blade support disk, with respect to the extension of the blades themselves. On the contrary, by propeller it is meant a rotary device in which the overall diameter is basically covered by the extension of the blades, rather than by the hub diameter. The devices of said PCT applications are intended for the propulsion of medium-speed (40÷60 knots) crafts and have an immersed blade crown sector of limited extension, so that the support disk is basically emerged.
This propulsion, in addition to suffering from intrinsic vibration problems and requiring complex adjustment mechanisms, is not suitable for speeds lower than 40 knots, in particular at cruising speeds deemed low (10÷30 knots). Another example, similar to the above one, is described in U.S. Pat. No. 6,851,991 (Eriksson), related to the use of paddle wheels running semi-submerged, built in the bottom and suitable for competition crafts.
The art is also aware of the use of the so-called semi-submerged propellers, also called surface-piercing. Such propellers run with the bottom half approximately immersed and the top half basically emerged. Thus, the side of the blades under negative pressure lies at a basically atmospheric pressure that prevents cavitation problems, making these propellers particularly suitable for very high rotation speeds, typical of variable-trim crafts, usually planing and characterised by high or very high speeds. Moreover, such an operation mode allows to keep emerged, i.e. “shaded” by the transom, the propulsion appendages (shafts and supports) at planing speeds.
This type of propellers have always been deemed unavoidably characterised by low propulsion efficiency, therefore totally unsuitable for crafts navigating at medium-low speeds. In particular, it is commonplace that such low propulsion efficiency could be favourably compensated for merely by the absence of immersed appendages at driving speeds. Therefore, the application range of these propellers has always been restricted to yachting and competition crafts or, in some sporadic instances, to cargo or military crafts, always characterised by high or very high operative speeds, usually in planing regimen. In their typical application, the semi-submerged propeller comes partially out of water only when the craft is in motion, whereas it remains completely immersed when the craft is not in motion.
Examples of this kind of application can be found in Italian Pat. No. 1,184,406 (Levi), in PCT Publ. No. WO-96/40550 (Arneson) and in U.S. Pat. No. 6,332,818 (Duncan et al.) describing a sort of hybrid between paddle wheel and surface-piercing propeller, fitted with a wide-diameter hub and short blades, spaced thereamong.